Flexible film bags are manufactured and sold for the packaging of a wide variety of products, especially food products such as meat. In use, the product, such as cut of fresh beef, ham, or pork, is placed into a bag having an open top and excess length (i.e., "bag tail"). After the product is within the bag, the atmospheric gases inside the bag are typically evacuated from the bag, and a heat seal is made across the top of the bag, near the product. After the seal is made across the top of the bag, the excess bag length (i.e., bag tail) is cut away from the packaged product and discarded or recycled. The film surrounding the meat is typically heat shrinkable, and is shrunk down against the meat product by passing the packaged product through a hot water tunnel, or other means for causing the film to shrink.
Beginning around 1976, the Cryovac division of W.R. Grace & Co. introduced the 8300 series automated rotary chamber vacuum bag packaging machine. This machine was and is successful in the marketplace. In operation, the 8300 machine clamped the bag between two clamps, punctured the bag, drew a vacuum within the bag, and then cut and heat-sealed the bag, to produce a packaged product in which the meat product was sealed within a vacuumized bag. Frequently, the bag was thereafter shrunk down tight against the meat product in a shrink tunnel, thereby further improving the appearance of the packaged product. The 8300 series machine performed the cutting and heat sealing operation by deploying a one-piece cutting/sealing member from a position above the film. The cutting/sealing member sealed the film by applying heat and pressure to the film as it held the film against a stationary seal seat located immediately on the other side of the film. Since by necessity the cutting knife impacted the film before the seal portion of the cutting/sealing member, the film was cut before it was held against the sealing member. As a result, due to the ballooning of the bag due to the relatively high pressure within the bag (due to the evacuation of the surrounding atmosphere in the chamber, and the relatively slow departure of air from within the bag), there was an undesirably high occurrence of bag "pull-back," in which, in spite of a clamp between the freshly-cut top of the bag and the product in the bag, the freshly-cut top of the bag was pulled back towards the product after cutting but before the sealing wire had a chance to clamp the bag film. The result of pull-back is a defective seal. The 8300 series machine also experienced some problems in accomplishing a complete cut-off of the bag tail, i.e., the excess portion of the bag to be removed and discarded. Partially-cut off bag tails occurred with greater frequency as the knife blade dulled from use. The partially-cut off bag tails had to be removed manually. In summary, the 8300 series machine exhibited some problems with incomplete bag tail removal as well as seal failures due to bag pull-back. Nevertheless, the 8300 series machine was the most advanced and commercially successful machine of its time, for the automated vacuum packaging of meat products in shrink bags. During at least the past 13 months, the 8300 series machine has been used for the packaging of various meat products in films in which a low Vicat Softening Point metallocene catalyzed film makes up at least 50 weight percent of the film.
Around 1986 the Cryovac Division of W.R. Grace & Co. introduced a next-generation automated rotary chamber machine for the vacuum packaging of meat products, i.e., the 8600 series machine. The 8600 series machine performed the same functions as the 8300 series machine. However, the 8600 series machine had a knife which was separately actuated from a dual clamp/seal-seat member. Moreover, the heat seal member was stationary, positioned below the film directly opposite the clamp/seal-seat member.
In the operation of the 8600 series machine, the clamp/seal-seat member was first deployed downwardly, from above the film. Thereafter, electrical current was supplied to a sealing wire in the heat sealing member, causing the film to be heat sealed. After the current to the seal wire was discontinued, the knife was actuated and the film was cut.
Compared with the 8300 series machine, the 8600 series machine exhibited fewer partially cut-off bag tails. This was due to the fact that the 8600 series machine did not exhibit seal failures due to bag pull-back, or partially cut-off bag tails, because the bag remained clamped by the dual clamp/seal-seat member while the knife made the cut, thereby making bag pull-back impossible, and reducing the number of partially cut-off bag tails.
In spite of the improvements provided by the 8600 machine, problems still existed. One problem was "edge tearing," in which the film "tears" at the seal, i.e., inward from the lay-flat edges of the bag. Edge tearing can be from either side of the lay-flat tubing (or in the center portion of the seal, rather than at the seal edges), and can extend only part of the way across the lay-fat tubing, or all of the way across the lay-flat tubing. If the tearing at the seal is so great that the tear extends the full length of the seal, the result is a problem called an "edge seal". While edge-tear and edge-seal occurred relatively infrequently when sealing traditional bags, recently when running bags containing film which was made from primarily homogeneous polymers (e.g., metallocene catalyzed polymers), and/or with bags made from thinner films, the edge tearing and edge sealing problems have become more frequent and more pronounced. It would therefore be desirable to reduce or eliminate edge-tear and edge-seal, especially for bags made from films containing primarily homogeneous polymers, especially homogeneous polymers which have not been irradiated, or which have been irradiated to only a low dosage level, and/or for bags made from relatively thin films.
The edge tearing and edge sealing problems become more frequent and more pronounced as the cutting knife becomes worn (i.e., more dull) from repeated use. The dull knife problem has been found to be significantly worse for films made from primarily homogeneous polymers (e.g., metallocene catalyzed polymers), and/or with bags made from thinner films. Although exchanging a dull knife for a sharp knife, or resharpening the knife, is a solution to the problem, it is costly, and is only a temporary solution. A better solution to this problem is desired.